The present disclosure belongs to a field of video surveillance technology, and more particularly relates to focusing under a point light source scene.
Under a point light source scene, the traditional auto-focus depending on image clarity evaluation may easily focus at a spurious crest of suspected peak, thereby leading to serious image blur.
More specifically, image clarity evaluation values may be acquired by using a clarity-evaluation-function, and the maximal image clarity evaluation value can be found by using some focusing search algorithms (e.g. the commonly used hill climbing search algorithm) based on the image clarity evaluation values, to determine the focus point position where the clearest image can be acquired. Under a normal focusing scene, with the focusing lens closing to or away from the focus point, the image clarity will monotonically increase or decrease, so that the image clarity evaluation curve demonstrates a better unimodality and the image clarity monotonically decreases on both sides of the peak value. For example, as shown in FIG. 1A, the horizontal axis denotes the position of the focusing lens and the vertical axis denotes the image clarity evaluation value. Under this condition, the optimum focus clarity point position can be determined by using the hill climbing search algorithm in combination with the appropriate search step and end conditions.
However, under a point light source scene, there may be spurious crest in the image clarity evaluation curve, for example, as shown in FIG. 1B, and may result in that the focusing search algorithm is trapped in a local extreme to lead to the defocus. Alternatively, under some point light source scenes, the image clarity evaluation curve may have the unimodality as shown in FIG. 1A, but there may be a deviation between the actual image clearest point position and the focus point position corresponding to the maximal image clarity evaluation value. Thus, searching for the focus point depending on the image clarity evaluation value, may easily search to a local extreme or lead to some repeat dither of the focusing effect, thereby affecting the accuracy and real-time of the focusing effect.